Patients undergoing cardiac surgery with the use of the cardiopulmonary bypass (CPB) machine have a very high (50-90%) incidence of pulmonary complications in the postoperative period. The exact etiology of these complications, which range from segmental atelectasis to pulmonary edema is not well identified. With this study we are testing the hypothesis that this alveolar- capillary membrane integrity can be better preserved in the patients undergoing CPB by ventilating lungs with 4% C02-containing gases during bypass and weaning phases to prevent severe alkalosis which may be the source of injury at this level. This is the first study designed to preserve the pulmonary functional integrity during the CPB phase and is being submitted for review by the IRB prior to actual study.